The instant disclosure relates to an integrated video, voice, and data communications system. More particularly, the present disclosure relates to a fiber-to-the-home/curb (FTTx) network system that is capable of propagating returning RF terminal signals from a subscriber to a data service provider.
The increasing reliance on communication networks to transmit more complex data, such as voice and video traffic, is causing a high demand for bandwidth. To resolve this demand for bandwidth, communications networks are relying upon optical fiber to transmit this complex high bandwidth data. Conventional communication architectures that employ coaxial cables are slowly being replaced with communication networks that comprise only fiber optic cables. One advantage that optical fibers have over coaxial cables is that a much greater amount of information can be carried on an optical fiber.
While the FTTx optical network architecture has been a dream of many data service providers because of the aforementioned capacity of optical fibers, implementing the FTTx optical network architecture may encounter some problems associated with legacy systems that are in current use by subscribers. For example, many subscribers of data service providers use set top terminals (STTs) to receive and transmit information related to video services. In some existing networks, the conventional set top terminals are coupled to a coaxial cable. The coaxial cable, in turn, is then connected to fiber optic cables in a hybrid fiber-coax (HFC) system. The coaxial cable from the set top terminals in combination with the fiber optic cables provide a two way communication path between the set top terminal and the data service hub for purposes such as authorizing a subscriber to view certain programs and channels.
For example, conventional set top terminals coupled to coaxial cables may provide impulse pay-per-view (PPV) services. Impulse pay-per-view services require two way communications between the set top terminal and the data service provider. Another service that requires two-way communication passed between the set top terminal and the data service provider is video-on-demand (VOD) services.
For video on demand services, a subscriber can request a program of his choosing to be played at a selected time from a central video file server at the data service hub. The subscriber's VOD program request is transmitted upstream on a return path that comprises coaxial cables coupled to fiber optic cables. With the VOD service, a subscriber typically expects VCR-like control for these programs which includes the ability to “stop” and “play” the selected program as well as “rewind” and “fast forward” the program.
In conventional HFC systems, a return RF path from the subscriber to the data service hub is provided. The RF return path is needed because a conventional set top terminal modulates its video service upstream data onto an analog RF carrier. While the video service upstream data may be modulated onto an RF carrier, it is recognized that the upstream data may be in digital form.
An RF return path comprises two-way RF distribution amplifiers with coaxial cables and two-way fiber optic nodes being used to interface with fiber optic cables. A pair of fiber optic strands can be used to carry the radio frequency signals between the head end (HE) and node in an analog optical format. Each optical cable of the pair of fiber optic strands carries analog RF signals: one carries analog RF signals in the downstream direction (toward the subscriber) while the other fiber optic cable carries analog RF signals in the reverse or upstream direction (from the subscriber). The high speed (Internet access) data service uses the same type of transmission with downstream RF modulated signals and upstream TDMA RF modulated burst mode signals. The subscriber premise equipment in this case is the Cable Modem.
Unlike HFC systems, conventional FTTx systems typically do not comprise a return RF path from the subscriber to the data service hub because the return paths comprise only fiber optic cables that propagate digital data signals as opposed to analog RF signals. In FTTx systems, a downstream RF path is provided because it is needed for the delivery of television programs that use conventional broadcast signals. This downstream RF path can support RF modulated analog and digital signals as well as RF modulated control signals for any set top terminals that may be used by the subscriber. However, as noted above, FTTx systems do not inherently provide any integrated capability of supporting a return RF path for RF analog signals generated by the legacy set top terminal. However, in order to provide PPV or VOD types of services, a secondary RF return path must be provided.
In prior art systems, multimedia service providers with an existing bi-directional data network have attempted to provide an RF return path overlayed onto the existing two-way data (time division multiplexed on same wavelength as upstream data). The upstream RF data is separated from the RF video signal at the subscribers home and overlayed onto an existing optical data stream being communicated to the service provider as part of a data service.
In more recent FTTx systems 10, such as shown in FIG. 1, the upstream RF signal is converted to a separate optical signal (different wavelength) (RF Return ONT) 12 at the subscriber's optical network termination device (ONT) 14, and is carried optically, through a local node 16, to a hub 18, with all of the other data streams on the same fiber. This all-optical network existing between the subscriber and the node 16 or hub 18 is called a passive optical network (PON). The PON comprises a point-to-multipoint downstream fiber network and a multipoint-to-point upstream fiber network. The current implementation of PON architecture requires four different data streams operating on four different optical wavelengths to achieve two-way broadband data, downstream video, and upstream video control.
FIG. 2 illustrates the wavelengths of the different optical signals within the PON. Downstream video (broadcast sub-carrier modulation SCM) (C-band) 20 is transmitted between 1525 nm and 1565 nm while upstream video control data (return SCM) (L-band) 22 is transmitted at about 1570 nm. For broadband data, the downstream data signal is transmitted between 1400 nm and 1500 nm (S-band) 24 while the returning upstream data signal is transmitted at a wavelength below 1400 nm (O-band) 26.
Now turning to the issue of the disclosure. The key to successful low-cost implementation of a PON is to eliminate any electronic components in the network pipleline. In this regard, the aggregated bi-directional optical signals travelling between a subscriber's ONT 14 and the local network hub 18 travel on optical fiber. The distance between a subscriber's ONT and the service provider's hub is typically about 5 to 20 kilometers. The network is laid out in such a manner that the optical components at the subscriber's ONT 14 and the service provider's hub 18 are sufficiently powerful to transmit and receiver the optical signals over the distance therebetween. However, once the optical signals reach the hub 18 there are several issues with which the service provider must contend. The first issue is amplification. The distance between hubs 18 can be much longer and the optical signals must be amplified to reach the next hub 18.
For purposes of the present disclosure, we are going to broadly identify that once the aggregated optical signals reach the service provider hub 18, the two broadband data signals are separated (by WDM splitters 28,30) from the two RF video signals and that the broadband optical signals and the video optical signals are handled along separate parallel paths (OLT 32). Amplification of the broadband data is handled separately and will not be discussed in detail other than to say that the data network systems inherently provide for two way amplification and transmission of the optical data signals as the broadband data system is intended to be a two-way data stream.
However, on the other hand, the video portion of these systems was not originally intended to be a two-way system. Video, until recently, was a one-way broadcast system. The hub systems are typically provided with a downstream amplifier (EDFA or erbium doped fiber amplifier) 34 for amplifying the downstream video signal. EDFA's are well known in the communication art for amplifying optical signals in the 1550 nm optical transmission window. The existing service provider hubs 18 do not have a way to transmit the upstream return SCM signal 36 (now in optical format) for transport back to the central office (CO) or head end (HE). Again referring to FIG. 1, in order to get the return SCM signal 36 back to the HE 38, the service providers have outfitted the existing hubs 18 with optical to electrical converters (O/E) 40 which receive the optical signal, convert it back to an electrical signal, and transmits the electrical signal back the HE 38 on existing twisted pair Gigabit Ethernet (Gbe) cables 42. The existing fiber optic backbone typically includes twisted pair lines, which can carry an electrical signal back to the HE. These retrofitted prior art systems require a plurality of optical receivers, associated O/E converters, and associated Gbe network adapters, all for the purpose of carrying back control signals for VOD and PPV. Needless to say, it is obvious that the fixed costs for providing all of this extra electronic equipment at each hub, not to mention the ongoing costs for powering and maintaining all of the extra equipment is quite high. Another similar system with multiple hubs is shown in FIG. 3 where it can be seen that each hub has a separate Gbe feed 42 back to the HE 38. It is also noted that the HE also requires the same type of electrical equipment to receive the multiple data streams.
Although effective, the multitude of optical receivers, O/E converters, E/O converters, analog to digital converters (ADC), digital to analog converters (DAC), GBe transmitters and Gbe receivers make transport of the return RF signal to the HE highly complicated and very expensive.
Accordingly, there is a need in the art for a system and method for communicating optical signals between a data service provider and a subscriber that eliminates the use of electrical wires and the plethora of related electronic hardware and software necessary to support the data signals propagating along the electrical wires. There is also a need in the art for a system and method that provides a return path for RF signals that are generated by legacy video service terminals (cable modems). Another need exists in the art for supporting legacy video service controllers and terminals with an all-optical network architecture.
The instant disclosure provides an all-optical video communication network where the upstream RF data signal remains in optical form until it reaches the HE.
As described above, the return RF signal is converted to an optical signal at the subscriber ONT and transmitted over the existing PON to the hub. However, in the improved system as taught by the disclosure, the hub is now provided with an upstream amplifier that will split off the upstream RF signal (L-band) from the rest of the signal, combine it with an upstream transport signal and amplify the combined signal for transport to the next hub, where it will eventually reach the HE.
As is understood in the art, the downstream SCM video signal (C-band) is at a substantially constant bias level and can be amplified uniformly using a conventional fiber amplifier. However, the upstream RF signal is a combination of a plurality of time division multiplexed (TMD) burst mode signals from the various ONT's, each originating from a different location at a different distance from the hub. Since optical signals are known to degrade over distance, particularly in a PON, the amplitude of each burst is different as it reaches the hub. For example, a burst originating from an ONT 5 km from the hub will be much higher than a burst originating from an ONT 20 km from the hub. Therefore certain accommodations must be made to equalize the incoming burst signals from the different PON's as well as to equalize the incoming burst signals with the overall transport signal with which it is being combined.
According to the disclosure, the upstream amplifier is provided with a saturating pilot tone, which establishes a consistent reference baseline for the signal and for amplification, and allows unlimited combination of the upstream signal.
More specifically, the upstream burst mode amplifier system includes a transport fiber configured and arranged to receive an upstream optical transport signal. An optical source (laser diode) is coupled to the upstream transport fiber wherein the optical source is configured and arranged to generate a saturating optical input signal that is combined with the optical transport signal to establish a baseline reference level for the optical transport signal. The upstream amplifier further includes a PON input configured and arranged to receive an incoming WDM optical signal from a PON. The incoming WDM optical signal includes the PON return RF control signals. A WDM splitter coupled to the PON input fiber splits the PON return RF control signals from the incoming WDM optical signal so that it can be input into the amplifier blocks. An optical amplifier is coupled to the output of the WDM splitter to pre-amplify the PON RF control signals and equalize the PON RF control signals with the power level of the optical transport signal as now established by the saturating input signal. A beam combiner combines the PON RF control signals with the optical transport signal and they are input into a second optical amplifier to amplify the optical transport signal, which now includes the pre-amplified PON RF control signals. A second optical source generating a second saturating input tone for the incoming PON signals can also be implemented to equalize the incoming PON RF signals prior to amplification.
By setting a baseline reference signal for the upstream transport signal, as well as equalizing and amplifying the PON return RF signals with the transport signal, the system provides for a consistent and stable upstream return path which can remain in all optical format until it reaches the head end (HE). Further by combining the upstream amplifier with a downstream amplifier in a single amplifier block, the disclosure allows the service provider to eliminate virtually all of the electronic system components from the network, providing a significant cost savings in equipment and ongoing power and maintenance costs.
Accordingly, it is an object of the present disclosure to provide for a system and method for communicating optical signals between a data service provider and a subscriber that eliminates the use of electrical wires and the plethora of related electronic hardware and software necessary to support the data signals propagating along the electrical wires.
Other objects, features and advantages of the disclosure shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.